The present invention generally relates to photovoltaic cells and, more specifically, to an enhanced multi-junction photovoltaic cell that includes buffer layers for the growth of single crystal boron compounds.
The interest in photovoltaic (PV) cells or solar cells has been increasing due to concerns regarding pollution and limited available resources. This interest includes both terrestrial and non-terrestrial applications. In the non-terrestrial environment of outer space, the concern over limited resources of any type is a major one because the need to increase an amount of a resource increases a spacecraft's payload. An increased payload can increase the cost of a launch more than linearly. With the ready availability of solar energy in outer space for a spacecraft, such as a satellite, the efficient conversion of solar energy into electrical energy is an obvious alternative to an increased payload.
The cost per watt of electrical power generation capacity of photovoltaic systems is a main factor that inhibits their widespread use in terrestrial applications. Conversion efficiency of sunlight to electricity is of critical importance for terrestrial PV systems, since increased efficiency usually results in a reduction of related electricity generation system components (such as, cell area, module or collector area, support structures, and land area) for a given required power output of the system. For example, in concentrator photovoltaic systems which concentrate sunlight from around 2 to around 2000 times onto the PV cell, an increase in efficiency typically results in a proportionate reduction of an area comprising expensive concentrating optics.
Irrespective of the application, and as with any energy generation system, efforts have been ongoing to increase the output and efficiency of PV cells. In terms of output, multiple cells or layers having different energy bandgaps have been stacked so that each cell or layer can absorb a different part of the wide energy distribution of photons in sunlight. The stacked arrangement has been provided in a monolithic structure on a single substrate or on multiple substrates.
In a multiple cell device, semiconductive materials are typically lattice-matched to form multiple p-n (or n-p) junctions. The p-n (or n-p) junctions can be of the homojunction or heterojunction type. When solar energy is received at a junction, minority carriers (i.e., electrons and holes) are generated in the conduction and valence bands of the semiconductor materials adjacent the junction. A voltage is thereby created across the junction and a current can be utilized there from. As the solar energy passes to the next junction, which can be optimized to a lower energy range, additional solar energy at this lower energy range can be converted into a useful current. With a greater number of junctions, there can be greater conversion efficiency and increased output voltage.
Whether in the multiple-junction or single-junction PV device, a conventional characteristic of PV cells has been the use of a single window layer disposed on an emitter layer which is disposed on a base layer. Further, the base layer may be disposed on a back surface field layer which is disposed on a substrate. The window layer and the back surface field layers are of higher bandgap semiconducting material lattice matched to the whole structure. The purpose of the top window layer and the back-surface field layer have been to serve both as a passivation layer and a reflection layer due to high electric fields associated with the high bandgap. The photo-generated carriers, such as the electrons in the emitter layer and the holes in the base layer, can further be reflected towards the p-n junction (which is the emitter and the base layer interface), for recombination and for generating electricity.
For a multiple-cell PV device, efficiency is limited by the requirement of low resistance interfaces between the individual cells to enable the generated current to flow from one cell to the next. Accordingly, in a monolithic structure, tunnel junctions have been used to minimize the blockage of current flow. In a multiple wafer structure, front and back metallization grids or contacts with low coverage fraction and transparent conductors have been used for low resistance connectivity. Since the output power is the product of voltage and current, a multi-junction solar cell can be designed with multiple junctions comprised of materials having different bandgaps, so that each junction can absorb a different part of the wide energy distribution of photons in sunlight. Additionally, uniform current generating characteristics may be produced.
Materials for a solar cell are conventionally grown epitaxially in a metal organic vapor phase epitaxy (MOVPE) system, also known as a metal organic chemical vapor deposition (MOCVD) system. During material growth, the lattice parameter for all of the different cell layers comprising the solar cell should be the same as that of the substrate. III–V compound materials of different compositions, but with the same lattice parameter as that of the substrate, are used to achieve different bandgaps that are typically required for multijunction solar cells. These layers are usually grown on a III–V substrate such as a GaAs wafer. In order to reduce the cost of the substrate material as well as to increase the over all power to weight ratio from the solar cell, a GaAs nucleated Ge substrate can be used. The lattice parameter of the Ge substrate is about 5.64613 Angstroms (Å) and that of GaAs is about 5.6533 Å with little mismatch between the lattice parameters. Although the Ge atomic structure is of a diamond structure pattern and that of GaAs is of a zinc-blend structure, it can be possible to grow GaAs on Ge with minimum defects. For a multijunction solar cell device, a thin layer of GaAs is first grown on the Ge substrate and followed by the growth of various other compositions.
Existing III–V semiconductor multi-junction solar cells are processed from epitaxial gallium indium phosphide/gallium arsenide (GaInP2/GaAs) materials, grown on a GaAs nucleated Ge substrate. By providing active junctions in GaInP2, GaAs, and Ge, a triple-junction solar cell can be processed. These existing triple-junction solar cells have demonstrated a 29.3% efficiency under space solar spectrum that is Air Mass 0 (AM0), 0.1353 W/cm2 at 28° C. Under the concentrator terrestrial spectrum (AM1.5D, 44W/cm2, 25° C.), an efficiency of 32.3% has also been demonstrated. The Air Mass value indicates the amount of air in space while the conversion efficiency describes a percentage of conversion from the sun's energy to electrical power. A limitation of such triple-junction solar cells includes the inability of increasing the AMO efficiency above 29.3% (to, for example, 35% or higher). To achieve such an increase, four junctions may be needed to enhance the utilization of the sun's energy spectrum.
Conventional methods to grow a triple-junction solar cell typically use GaInP2, GaAs and Ge cells. The direct bandgaps of GaInP2 and GaAs are about 1.85 eV and about 1.424 eV respectively (Ge has an indirect bandgap of about 0.66 eV). Theoretical studies have shown that an additional third junction of about a 1.0 eV solar cell disposed on top of the Ge junction may be necessary for building a four junction monolithic solar cell. As such, GaInP2 may form the first junction, GaAs can form the second junction, a new 1 eV material may form the third junction and Ge can form the fourth junction. Limitations of such materials include a lack of a bandgap around 1.0 eV that may be lattice matched to Ge and a lack of requisite material properties needed to process a solar cell. Some materials such as Gallium Indium Arsenic Nitride (GaInAsN) have been used in an attempt to achieve lattice-matching characteristics, however an ability to produce material with requisite characteristics and with a bandgap around 1.0 eV has not been achieved.
Using boron compounds may allow the formation of a junction material having a bandgap of about 1.0 eV in a multi-junction solar cell thus resulting in a higher voltage output. Prior art methods to grow epitaxial single crystal boron compounds, such as those from National Renewable Energy Laboratories (NREL), have followed conventional MOCVD growth techniques to achieve a low boron content of about 1.6% and could not demonstrate lattice matched boron based materials (such as BGaInAs) with high boron content due to a reaction between indium and boron source gases during the growth of the boron based layer. Further some prior art techniques to re-crystallize the material from a polycrystalline state to a monocrystalline state include annealing, high temperature melting and solidifying. None of these techniques, however, could form epitaxial boron compounds.
The conventional MOCVD process for growing a layer on a substrate requires a thermal heat clean of the substrate to remove the surface oxides. Then, a standard buffer layer, such as a GaAs layer on a GaAs substrate, can be grown at the same growth temperature as the layers grown above it. NREL has followed this process to grow a GaAs buffer layer and BGaAs or BGaInAs test layers above the GaAs layer. Utilizing such a traditional method, NREL could not achieve a preferred composition (B0.093 Ga0 707 In0 2 As, further described below) in the BGaInAs layer to produce an about 1.0 eV bandgap due to crystallization problems in enhancing the boron content as well as the pre-reaction problems associated with the source materials.
U.S. Pat. No. 6,281,426 B1, assigned to Midwest Research Institute, describes utilizing GaInAsN for the third junction in a four junction solar cell in an attempt to provide a bandgap of about 1.0 eV following the traditional growth method described above. Other materials, such as BGaInAs and GaAsGe, were also presented for forming the third junction solar cell. However, because the traditional growth method was utilized, the required composition of BGaInAs material to achieve an about 1.0 eV bandgap was not realized.
As can be seen, there exists a need for an improved multi-junction photovoltaic cell having a 1.0 eV bandgap with requisite properties of single crystal boron compounds without the problems associated with a pre-reaction of source gases and of single crystal formation.